Electronic ballasts generally include an inverter that provides high frequency current for efficiently powering gas discharge lamps. Inverters are commonly classified according to switching topology (e.g., half-bridge or push-pull) and the method used to control commutation of the inverter switches (e.g., driven or self-oscillating). In many types of electronic ballasts, the inverter provides an output voltage that is processed by a resonant output circuit to provide a high voltage for igniting the lamps and a magnitude-limited current for powering the lamps.
When the lamps fail, are removed, or otherwise cease to operate in a normal fashion (e.g., such as what occurs during so called “diode mode” operation), it is highly desirable that the inverter be shut down or at least shifted to a different (e.g., low power) mode of operation. This is necessary in order to minimize power dissipation, reduce heating in the ballast, and protect the components of the ballast from damage due to excessive voltage, current, and heat. Circuits that shut down or alter the operation of the inverter in response to a lamp fault condition are customarily referred to as lamp fault protection circuits.
Many of the existing approaches to lamp fault protection rely upon detecting asymmetries in either the lamp voltage or the lamp current as an indication that a lamp fault condition has occurred. For example, U.S. Pat. No. 5,777,439 discloses an arrangement wherein rectifying devices are connected to both sides of a lamp in order to detect an unbalanced voltage (which is known to be indicative of at least certain types of lamp failure modes, such as diode mode operation). Such a fault detection approach is effective and economical for ballasts that have a non-isolated output (i.e., no output transformer interposed between the resonant output circuit and the connections to the lamps), but it is not well-suited for those types of ballasts (e.g., a ballasts that includes a current-fed self-oscillating inverter and a parallel resonant output circuit with an output transformer) that inherently provide electrical isolation between the ballast circuitry and the connections to the lamps. For the latter type of ballasts, any signal indicative of a lamp fault condition must be transferred from the secondary side of the output transformer to the primary side via a suitable isolation device, such as an optocoupler, before it can be used to control operation of the inverter. As can be expected, the added presence of an isolation device increases the cost and complexity of the ballast.
Many existing ballasts with lamp fault protection circuits respond to a lamp fault condition by shutting down the inverter and then keeping the inverter off for as long as power continues to be applied to the ballast. With such ballasts, following replacement of a failed lamp with an operational lamp, it is required that power to the ballast be turned off and then on again (i.e., “cycled”) in order to effect ignition and powering of the lamps in the fixture. This requirement poses a considerable inconvenience in many applications, such as in large office areas or factories, in which a large number of ballasts are often connected in the same branch circuit. In such environments, with many existing ballasts, it is necessary to momentarily interrupt the lighting in a large area in order to restore desired operation to even a single lighting fixture after one or more of its lamps are replaced. It is thus desirable to have a ballast that accommodates relamping without requiring that the power to the ballast be removed and reapplied.
It is important that lamp fault detection be inhibited during certain operating periods, such as inverter startup and lamp ignition. For instance, the normal starting process of the inverter and lamps is generally accompanied by the same types of electrical disturbances that occur during a lamp fault condition. Thus, unless lamp fault detection is inhibited during inverter startup and lamp ignition, the inverter may be preventing from properly starting and/or the ballast may be prevented from properly igniting the lamp. Additionally, although most lamps are capable, under ideal conditions, of igniting and operating normally within a short period of time (e.g., 20 milliseconds), some lamps, due to age or low temperature, require a much longer time to ignite and stabilize. Thus, lamp fault detection should be inhibited for a period that is long enough (e.g., at least 200 milliseconds or so) to accommodate lamp starting under conditions that are less than ideal.
It is desirable that a ballast possess some type of automatic restart capability wherein, within a specified time following detection of a lamp fault condition and shutdown of the ballast, periodic attempts are made to restart the ballast and ignite the lamp. This feature is desirable in order to prevent a “latched” shutdown of the ballast (which necessitates that power to the ballast be turned off and then on again in order to reset the ballast) in the event of false detection due to a momentary power line transient or any of a number of anomalous phenomena (e.g., electrical noise) that pose no real threat to ballast reliability or safety. Also, because lamps are somewhat unpredictable, it is possible that an otherwise “good” lamp may sometimes fail to properly start on the first attempt. In such a case, a ballast with automatic restart capability will periodically attempt to start the lamp, rather than simply latching the ballast or its inverter in a shutdown state until such time as the power to the ballast is cycled.
For ballasts that power multiple lamps and that includes automatic restart capability, in the event of a sustained lamp fault condition (i.e., a lamp fault condition that remains present for an extend period of time such as, e.g., hours, days, weeks, months, etc.), the periodic (but unsuccessful) attempts to restart the ballast and ignite the lamps results in a regular (e.g., once per second) brief flashing of any remaining operational lamp(s). This regular brief flashing, which occurs on a sustained basis until either the lamp fault condition is corrected or power is removed from the ballast, is considered to be visually annoying to occupants who are in the vicinity of the affected lighting fixture. Additionally, the periodic restart attempts are stressful to the components within the ballast. Thus, a need exists for a lamp fault protection approach that not only minimizes visual annoyance to occupants, but that also avoids placing unnecessary stress upon the ballast components.
Ballasts for gas discharge lamps provide high ignition voltages for starting the lamps. The ignition voltages supplied by preheat type ballasts are typically on the order of several hundred volts (e.g., 500 volts peak), while those provided by instant-start type ballasts may exceed 1000 volts peak. As a consequence of these high ignition voltages, ballasts are subject to a special type of lamp fault condition that is commonly referred to as output arcing.
Output arcing may occur in any of a number of different ways. For example, in fluorescent lighting installations, it is a common practice to replace failed lamps while AC power is applied to the ballast. This practice is referred to as “live” relamping. During live relamping, as a lamp is being removed or inserted, a momentary arc may form between the fixture socket contacts and a pin of the lamp. As another example, a sustained arc (as opposed to a momentary arc) may occur due to poor or faulty connections in the output wiring or the lamp sockets, or if a lamp is improperly installed in such a way that a small gap exists between the lamp pins and the contacts within the fixture sockets. If a connection to a lamp is compromised due to a defective lamp socket or defective wiring, a high intensity, high temperature arc may be produced across the air gap caused by those faulty connections.
Arcing is generally acknowledged to cause degradation of the contacts in the fixture sockets and undue stress on components within the ballast. Sustained arcing is especially undesirable because of its tendency to produce potentially destructive heating. In order to minimize any ill effects due to arcing, it is important that the arc be promptly extinguished. This requires a ballast that is capable of quickly and reliably detecting an arc and, subsequently, taking appropriate action to promptly extinguish the arc.
The prior art includes a number of circuits for detecting and/or protecting against output arcing. A more thorough discussion of the problems and prior art relating to output arcing is provided in U.S. patent application Ser. No. 11/532,277 (titled “Ballast with Arc Protection Circuit,” filed on the same date, and assigned to the same assignee, as the present application), the disclosure of which is incorporated herein by reference.
Existing approaches for lamp fault protection are largely limited to protecting against more conventional types of lamp faults (e.g., failure or removal of a lamp, diode mode operation, and degassed lamp), and do not provide reliable protection against output arcing. Conversely, existing approaches for protecting against output arcing generally do not protect against more conventional types of lamp faults. Thus, a need exists for an approach that reliably combines protection against conventional lamp faults with protection against output arcing.
A significant shortcoming of many existing protection approaches is that the employed circuitry generally provides for “analog” type detection (as opposed to “digital” type detection). Besides being susceptible to problems attributable to electrical noise, analog approaches are generally sensitive to variations in component tolerances and operating parameters, and are therefore often quite lacking as to robustness. More particularly, analog approaches are generally incapable of compensating for the fact that significant parameters of the ballast circuitry and the lamps tend to vary with time and temperature. For example, it is well known that the operating voltage and current of a gas discharge lamp tends to significantly change as the lamp “warms up.” As another example, certain ballast components such as integrated circuits are prone to considerable parameter variations (e.g., in the frequency of the internal oscillator) as the ballast ambient temperature changes. In some applications, some of the deficiencies attributable to component tolerances and parameter variations can be reduced by design measures (e.g., using components with “tight” tolerances), but almost certainly at the price of substantial added cost to the ballast. As yet another example, the nature (e.g., lead length, spacing, etc.) of the wiring within the lighting fixture often exerts a significant effect (due to parasitic capacitances, and such) upon the operation of the ballast and lamps. Thus, a further need exists for a protection approach that is substantially insensitive to, or at least capable of largely compensating for, at least some of the more significant component tolerances and parameter variations, as well as sources of electrical noise, that are inherent in electronic ballasts, gas discharge lamps, and lighting fixtures, but that is still capable of being implemented in an economical and manner.
Yet another shortcoming of many existing approaches to lamp fault protection is that those circuits often require a considerable amount of operating power. Typically, the operating power requirements increase with circuit complexity, especially when analog circuitry is extensively employed. Consequently, those circuits significantly detract from the overall energy efficiency of the ballast. Thus, a further need exists for a lamp fault protection circuit that, in comparison with existing approaches, has relatively modest operating power requirements.
Ballasts with a current-fed self-oscillating inverter and a parallel resonant output circuit are currently the prevailing “instant start” design topology in North America. However, providing reliable lamp fault protection within these types of ballasts presents a significant engineering challenge. As previously discussed, existing approaches based upon analog detection have a number of deficiencies, including costly/complicated circuitry, susceptibility to electrical noise, sensitivity to parameter variations and component tolerances, and a need for providing electrical isolation between at least portions of the lamp fault protection circuit and the rest of the ballast circuitry. Additionally, many prior art approaches are susceptible to problems relating to detection resolution, and are therefore ill-suited for ballasts that power multiple (e.g., three or four) lamps. For example, in a ballast for powering three or four lamps and in the case of a lamp fault condition that involves only one lamp, any signal that is intended to be indicative of a lamp fault condition may be “swamped out” by the fact that the remaining two or three lamps are operating in a substantially normal manner. Because of this problem, one existing approach has been to provide a separate inverter and output circuit for each of the lamps powered by the ballast; such an approach has the obvious disadvantage of being quite expensive, especially for ballasts that power three or four lamps (in which case three or four separate inverters and output circuits are required).
Thus, a need exists for a ballast having a lamp fault protection circuit that, in addition to protecting against conventional lamp fault conditions, is capable of protecting against damage (to the ballast and its associated lighting fixture) due to the output arcing. A need also exists for a lamp fault protection circuit that is capable of detecting a lamp fault condition in a reliable manner and that is substantially insensitive to electrical noise and to problems/variances attributable to component tolerances, fixture wiring, number of lamps, and operating parameters within the ballast, lamps, and lighting fixture. A need also exists for a ballast and lamp fault protection circuit that provide a starting (i.e., inhibit) period in order to allow for proper lamp starting. A further need exists for a ballast and lamp fault protection circuit that provide automatic restart capability in order to accommodate false detection and anomalous starting failure of a “good” lamp. A further need exists for a ballast and lamp fault protection circuit that provide automatic restart capability, but that also minimize visually annoying flashing and unnecessary stress to ballast components when a lamp fault condition remains present for an extended period of time. A further need exists a lamp fault protection circuit with modest operating power requirements. A further need exists for a lamp fault protection circuit that provides all of the aforementioned functional benefits, and that is readily and economically implemented within existing ballasts. Such a ballast and lamp fault protection circuit would represent a considerable advance over the prior art.